This patent relates to a method of making a dry bonded paperboard structure. More particularly, this patent relates to a method of making a spirally wound paperboard container in which a radio frequency active adhesive is added to the pulp stock during the paperboard fabrication process.
Paperboard is one of two broad subdivisions of paper (general term), the other being paper (specific term). The distinction between paper and paperboard is not sharp, but, broadly speaking, paperboard is heavier, thicker, and more rigid than paper. For the purposes of this patent, the term xe2x80x9cpaperxe2x80x9d shall include paperboard and the term xe2x80x9cpaperboardxe2x80x9d shall include paper.
Paperboard can be used to make numerous structures, including spirally wound structures such as tubes, cores and cylindrical containers. In the manufacture of spirally wound containers, a web of paperboard is fed at a desired angle to a stationary mandrel to form the structural or bodywall layer of the container. Prior to being wound, a permanent heat sensitive adhesive is applied by a roller along a marginal edge of the paperboard web. The web is passed under a heater that softens the adhesive and makes it tacky. As the web is wound around the mandrel, the first marginal edge of the web advances back under the mandrel and is brought into contact with the opposing edge of the ensuing portion of the web. The edges become adhered to form a spirally wound tube, which can then be cut into desired lengths.
The heat sensitive adhesive is usually aqueous based, of which a few examples are vinyl acetate/ethylene copolymers, polyvinyl alcohol, polyvinyl acetate (a.k.a. xe2x80x9cwhite gluexe2x80x9d), dextrin, casein and acrylics. The problem with using aqueous based adhesives is that the water from the adhesive can migrate into the paperboard, potentially decreasing the mechanical properties of the paperboard, such as compression strength, tensile strength, tearing strength and folding endurance.
Drummond et al. U.S. Pat. No. 6,296,600 discloses a method of reducing the migration of water into the paperboard by using a foamed adhesive, which reduces the amount of adhesive that comes into contact with the paperboard. While this solution may be effective, there still exists a need for a method of making paperboard containers that reduces or eliminates the amount of water migrating into the paperboard and at the same time strengthens the paperboard.
This need is solved by the present invention through the use of radio frequency (RF) active adhesives and RF heating. As is well known in the art, radio frequency heating is a method used to generate heat directly within a material containing RF active components (susceptors), and indirectly within materials that are in thermally conductive contact with RF susceptors. RF susceptors are ionic or polar materials that have the ability to convert RF energy into thermal energy when exposed to an RF electromagnetic or electrical field.
As disclosed in International Patent Application Nos. WO 99/47621 and WO 01/21725, RF active adhesives (adhesives containing RF susceptors) can be used to adhere two or more layers of non-conducting substrates. Adhesion is accomplished by exposing an adhesive layer between the substrates to radio frequency energy in the range of from about 1 MHz to about 100 MHz, which induces dielectric current in the RF susceptor. The current generates thermal energy, which causes the adhesive to soften and adhere the adjoining substrates.
These and other patents describe methods of bonding adjacent substrates by coating one or both substrates with an RF active adhesive. What is heretofore not known is that two or more paperboard layers can be bonded by adding RF active adhesive to the paperboard pulp stock during the paperboard making process.
Thus it is an object of the present invention to provide a method of making a paperboard structure that reduces or eliminates the amount of water migrating into the paperboard.
Another object of the invention is to provide a method of making a spirally wound paperboard structure that eliminates the step of coating the paperboard web with adhesive prior to winding.
Yet another object of the invention is to provide a method of making a spirally wound paperboard structure that uses an RF active compound for adhesion.
Still another object of the invention is to provide a method of making a multiple ply paperboard suitable for dry bonding in which an RF active adhesive is added to the outer plies of the paperboard but not the inner plies.
Further and additional objects will appear from the description, accompanying drawings, and appended claims.
The present invention is a method of making a dry bonded paperboard structure by adding an RF active adhesive to the pulp stock during the making of the paperboard. The method comprises the steps of adding a radio frequency active adhesive to paperboard pulp stock; forming the pulp stock into a web or sheet; exposing the sheet to RF energy to generate heat sufficient to cause the adhesive in the sheet to soften; arranging one or more of the sheets in at least partially overlapping relationship; and allowing the adhesive to harden, thereby forming the multiple-ply paperboard structure. The method may be used to make spirally wound tubular structures such as tubes, cores and cylindrical containers. By eliminating the need to coat the paperboard with aqueous adhesive, the method reduces or eliminates the migration of water into the paperboard, thereby producing a stronger paperboard structure.
In an alternative embodiment, the paperboard comprises multiple plies, and the adhesive is added only to the pulp stock that is used to make the outer ply or plies.